The present invention relates to transparent films and compositions, and in particular to transparent films for optical applications.
Polycarbonate (PC) is an engineering thermoplastic resin with excellent toughness and clarity. Due to its excellent performance characteristics, polycarbonate is used in many applications requiring optical quality, including compact disks, ophthalmic lenses, and as diffuser films in backlit display devices. Polycarbonate films offer a number of advantages over poly(ethylene terephthalate) (PET), films including higher heat distortion temperature (HDT), higher scratch resistance, and better performance under stringent environmental conditions.
One of the major performance demands of a diffuser film is dimensional stability. Generally, when polycarbonate films are used in this application, portions of the film near the hot lamp may expand, while film further from the lamp either does not expand or does not expand to a similar extent, resulting in warping or wrinkling in these portions of the film. This can lead to an optical waving effect that is evident by sinusoidal oscillations in luminance intensity across the display panel after exposure. This problem may become progressively worse as the area of the film increases, and as the environmental conditions become more stringent. For larger displays like those in liquid crystalline display television (LCD TV) there is a demand for more and brighter fluorescent lamps, hence exposing the back lit module (BLM) and display to higher temperatures.
There accordingly remains a need in the art for films that that will remain flat over a wide temperature range, i.e., films that are dimensionally stable and therefore non-waving in use. It would be a further advantage of the film maintained a combination of advantageous physical characteristics such as low haze, high percent transmission (% T), low yellowing, and/or improved melt stability. Such compositions can be used in making films that will retain flatness over a wide range of end use conditions.